1. Field of the Invention
The present invention concerns a coating material feeding apparatus for feeding a coating material prepared by mixing two or more kinds of coating material ingredients at a predetermined ratio, particularly, an aqueous two-component mixed coating material comprising a main agent and a curing agent to a coating machine or a coating material tank equipped with or detachably mounted thereto.
2. Statement of the Related Art
In recent years, with a view point of global economical preservation, regulations for organic solvents and VOC regulations of coating materials in coating processes have become severer and, in order to cope with such demands, aqueous coating materials not using organic solvents have been developed in the field of the coating industry and their markets have been extended.
In the coating of automobile bodies, among undercoatings, intercoatings and topcoatings, undercoatings have been opened usually by electro-deposition coating of aqueous coating materials, and most of organic solvent type coating materials used so far for the intercoatings have now been replaced with aqueous coating materials or powder coating materials.
Further, also for the topcoatings, almost of base coatings have been replaced with aqueous coating materials or powder coating materials except those for special colors. However, organic solvent type one-component or two-component mixed coating materials have to be used only for the clear coatings requiring higher quality, since aqueous coating materials capable of satisfying high coating quality in view of appearance, weather proofness, water proofness, chemical resistance, resistance to acid rains and scratch resistance are not present.
However, aqueous two-component mixed coating materials using a main agent and a curing agent in admixture have been developed recently as aqueous clear coatings of firm coating films having physical properties comparable with those of organic solvent type component mixed coating materials.
In the aqueous two-component mixed type coating material, a main agent comprising a water soluble or water dispersible polyol having hydroxyl groups as a base resin is mixed with a curing agent comprising a water dispersible polyisocyanate as a main ingredient and crosslinked and cured.
However, in the aqueous two-component mixed coating material of this kind, the water dispersible polyol as the main agent is hydrophilic whereas the polyisocyante as the curing agent is hydrophobic, so that they tend to be separated like water and oil to result in a problem that uniform mixing is difficult by merely interposing a static mixer in a coating material feed channel as in the case of the organic solvent type two-component mixed coating material.
Accordingly, materials previously stirred and mixed mechanically by a blender or the like are fed to a coating machine. However, in a case of continuous coating for a long time as in automobile coating, since the main agent and the curing agent start curing reaction upon mixing under stirring, the coating material is gradually cured during supply and the viscosity of the coating material changes to make the coating quality not constant, or coating material remaining in the coating material feed pipeline is cured to cause clogging, or it is discharged from the coating machine and deposited on the surface of the coating film to possibly result in coating failure of forming grits.
In view of the above, as a means for feeding the aqueous two-component mixed coating material under complete mixing, it may be considered a method of feeding and mixing the main agent and the curing agent each at a flow rate in accordance with the mixing ratio constantly and at a high pressure to a jetting diffusion mixer.
In this case, when a gear pump is used for the supply of the main agent and the curing agent each at constant amount, while the gear pump is excellent in the constant feeding performance at a low pressure, the main agent and the curing agent leak through gaps of the gear when a high pressure is exerted and constant feeding property can not be maintained.
Particularly, during long time use, the gear is worn to cause leakage, and the mixing ratio varies by the error in the flow rate, or worn metal powder of the gear intrudes into the coating material to possibly cause coating failure.
In addition, since the gear pumps for feeding the main agent and the curing agent have to be controlled individually at respective number of rotations previously set in accordance with the mixing ratio, the control is troublesome, as well as motors are necessary for individually driving the gears to result in a problem that the size of the apparatus is increased.
On the other hand, since a cylinder pump is excellent in the constant feeding property and durable also to a high pressure, the main agent and the curing agent of the aqueous two-component mixed coating material can be fed with no previous mixing, but by mixing them just before use.
In the actual lines, it is desirable that the control is extremely simple and compact so as not to in the way when installed in the coating line and, in addition, that the installation cost or running cost are inexpensive and the maintenance is easy.
Regarding this, there is still left problems to be solved, for example, as described below. That is, it is troublesome to arrange various kinds of pipelines such as pipelines for connecting each of the feed sources for the main agent and the curing agent with each of the cylinders, pipelines for guiding the main agent and the curing agent discharged from each of the cylinders to the mixer or the like and supply pipe lines and discharge pipelines for a hydraulic fluid that drives each of the pistons of the cylinders, or a number of valves are required for turning the pipelines on and off, which increases the number of parts and making control, assembling and maintenance operations troublesome.